Wednesday, 9 November 2011

Space exploration: present and future

by Chris North, School of Physics and Astronomy, Cardiff University

Silent Running was released at a time when there were almost no limits on the imagination of science fiction writers and screenwriters. The advent of the space age led to the belief that we really would be travelling through the Solar System in a few decades. Sure, there were technical challenges, but these were thought to be easily surmountable. Other films of that era, such as Arthur C Clarke's "2001: A Space Odyssey", treat the difficulties of space travel with some respect for the science, while others exercise a little more poetic licence. There's not necessarily anything wrong with that - talking about magnetic boots, gravity plating (whatever that is!) might detract from the main story - but the apparent disregard for these things does stick in the throat a little.

By 1972, when this film was released, the furthest we'd reached in terms of unmanned exploration was Mars, and the outer Solar system remained a mystery. The planet Saturn, around which much of the action takes place, has long been a point of fascination, with its majestic ring system setting it apart from the other planets. Since 1972, we've accomplished an awful lot and know an immense amount more than we used to about Saturn. In 1979 Pioneer 11 passed Saturn, and in the 1980s the two Voyager probes flew by and sent back stunning images of the ring system. More recently, in 2004, the Cassini probe arrived at Saturn and is still exploring it today. Some of the most amazing discoveries have been regarding the structure of the rings.

Flying through the rings of Saturn would not be anything like what was seen in the film, though since there had been no observations other than those from Earth this is completely forgivable. The rings are composed of tiny particles of ice, most of which are a fraction of a millimetre in size, though some are huge boulders. The main part of the rings are around 300 thousand km across from side to side, though there are more tenuous rings several times that diameter. What is truly astonishing is their thickness, which was a huge surprise when it was measured. They are one 10 metres or so thick! Flying through them would be fairly damaging to a spacecraft, which would likely find boulders flying towards them at immense speeds, though not for very long. Even the tiny particles are damaging at these speeds which can reach tens of thousands of km per hour.

Of course, manned exploration of the Solar System has yet to begin, with the Moon being the furthest mankind has reached. While there are plans for future missions to Mars and some asteroids, these are still decades away. But we are beginning to understand what the most significant challenges might be, and how they might be addressed. There are some aspects of the film that I think are probably more accurate to manned exploration.

Firstly, everything tends to take place really slowly. That's not because the speeds are low, but because the distances are so immense. It's around a 1.5 billion km (1 billion miles) out to Saturn, and this journey took the Cassini probe 7 years to complete. It didn't go directly there, using gravitational sling-shots of Venus, Earth and Jupiter to give it enough speed to reach Saturn. Of course, in the film they talk about a 6 month journey, but it could be that they have much more powerful engines than we have today. Being so far away causes problems for communication. Radio transmissions, which travel at the speed of light, take around an hour to travel from Earth to Saturn, so meaningful conversation is all but impossible. The fact that they simply get transmissions from Earth is realistic. Of course, the ships would not be controlled from Earth. As indicated in the film, future exploration would rely heavily on computer control.

There would certainly be robots to fix things, as going outside would be a last resort for the people. That's not because of the hassle of donning space suits, but rather the problem of radiation from the Sun. The high energy particles that make up the solar wind from the Sun are the greatest challenge for manned exploration of the Solar System, and any future spacecraft would have to be very heavily shielded. The robots would most likely not look like Huey, Dewy an Louie, though, as there would be a huge range of maintenance robots to do specific jobs, from adjusting components on the outside of the ship to cleaning the waste pipes. I doubt that they will be at all humanoid in appearance, though it's possible that there'll be humanoid robots to help with the ironing and hoovering on board...

Of course, robots need power, as does the rest of the ship. Out at the orbit of Saturn, the Sun is 100 times weaker than it is at Earth, and so solar power really isn't feasible. They don't specify what propulsion method is used on board, though much of the literature of the time assumed we were a few decades off having nuclear fusion power - which is coincidentally how far off we probably are today! Unmanned probes such as Cassini and the Voyager probes use radioisotope thermal generators, which draw power from the radioactive decay of plutonium.

Obviously travelling to Saturn and back requires a significant amount of fuel. The ship seems to be carrying large tanks, so perhaps it was assembled in Earth orbit with large fuel tanks attached. This is possible, but probably not the most efficient way of travelling. Today's spacecraft predominantly rely on normal chemical rockets, though there are other forms of fuel available. The Dawn mission to the asteroids Vesta and Ceres uses an ion drive. Instead of sending the products of a chemical explosions out of the exhaust, Dawn sends a stream of very energetic particles. This produces a very low thrust, but it is very efficient and can burn for a long time. The efficiency is crucial, as a huge challenge for interplanetary travel would be carrying enough fuel to accelerate and decelerate such a large spaceship.

There are other methods of propulsion that are envisaged as well. Rather than chemical explosions, it might be possible to use nuclear explosions to push the ship forwards. Risky, and certainly not advisable in the Earth's atmosphere, but a method that was considered plausible for a long time. Another form of propulsion is a Solar Sail, which only works for very light craft, but requires no fuel. It simply uses the pressure from the Sun's light to push a very lightweight film. It has been demonstrated by both NASA and the Japanese space agency, and may well become the favoured method of propulsion for journeys that do not require high speeds.

In "Silent Running" the ships are meant to be commercial ships which have been used to transport forests, so perhaps they were mining vessels. Maybe they collected fuel in the asteroid belt on their way out to Saturn. Or perhaps they were gong to harvest Saturn's rings to generate fuel from the water ice. I think that the commercial exploitation of space travel is what is really required to make it commonplace. At the moment, Virgin Galactic are offering tickets for 5 minutes of weightlessness for around £100,000, which means it is limited to the very rich and famous. But look at normal air travel, and how quickly that industry has grown in just a century. Things like mining will likely be completely robotic, but I find it hard to believe that we won't eventually have colonies on the Moon and Mars. Perhaps they'll become holiday destinations for the rich and famous, and then eventually build up their own communities. How long will it be before we can get budget flights to Mars with "easyrocket"?

So now the money question: how far off are we? We've proved that we can send people to the Moon, and build laboratories in orbit around the Earth. But for 40 years we've not travelled further than a few hundred kilometres above the Earth's surface. To make matter worse, America's manned spaceflight programme is experiencing a short hiatus after the end of the Space Shuttle programme. They are relying on Russian launches to send astronauts to the International Space Station. In a few years, it is possible that we'll have the first commercial launches of manned spacecraft, from companies such as Space X and their Dragon capsule. NASA are developing a multipurpose crew transport vehicle, which has more in common with the Apollo rockets than the Space Shuttle.

So where to send the next wave of explorers? We've done the Moon, though we could have a practise at setting up a base, possibly near the lunar south pole. China, who are in the process of building a space station in orbit, also have designs on the Moon. Mars is also quoted as being a future destination, though that would involve a mission lasting at least two years and would require many of the current challenges to be overcome, such as the production of fuel on another planet and the shielding from radiation.

The next target for America is currently a near-Earth asteroid. Not necessarily to land on it, but just to "interact" with it. This sounds like an easy challenge, but in fact poses many difficulties that don't exist with missions to the Moon and Mars. It involved operating in deep space, far from the gravitational pull of a planet or Moon. We've shown that people can live and work in Earth orbit, and even on the Moon, but any interplanetary missions of the future will spend the vast majority of their time between the planets. If we can travel to a near-Earth asteroid and back, then perhaps we can send astronauts to the "L2" point, the gravitational sweet-spot where satellites such as Herschel, Planck and (in a few years) the James Web Space Telescope operate. This is a million miles away, and currently well beyond our reach. If we can get to the stage of sending astronauts to work and maybe even live there, then we have a hope of one day building the sort of infrastructure required to taken people to the other planets.



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